1. Field of the Invention
The present invention relates to the field of combustion technology. It concerns a device for measuring flame temperature.
2. Background of the Invention
The determination of flame temperature has been attributed great importance since the start of research in the field of combustion technology. Flame temperature is a key parameter in the combustion of fossil fuels, since it is directly correlated with the chemical reaction kinetics and the formation of pollutants such as, for example, NOx. Moreover, knowledge of the release of energy during the combustion process is indispensable for the design of combustion chambers and determination of mechanical and the thermal loads of all components concerned.
At present, there are a multiplicity of techniques for measuring flame temperatures. However, the extreme operating conditions in this case represent a great challenge to the temperature sensors, with the result that it is not directly possible for every temperature sensor tested under clean laboratory conditions to be used in an industrial combustion chamber.
Broadly speaking, the temperature-measuring techniques commonly used today can be divided into two categories; non-optical temperature sensors are used in the first ones, and optical sensors are used in the others.
Point sensors, which comprise thermocouples, for example, belong to the non-optical temperature-measuring devices. They offer a simple and inexpensive possibility for determining temperature at discrete points, but they must be installed in the direct vicinity of the flame and therefore influence the flame. Furthermore, because of their fragility, thermocouples can be used only to a limited extent in a turbulent high-temperature environment in which, in addition, chemical surface reactions further impair the thermocouples.
Particularly since laser technology has become known, numerous optical temperature-measuring devices have been developed. These include, inter alia, absorption and fluorescence techniques as well as various measuring techniques employing scattered laser light. The said optical measuring methods have in common that they require a light source, a laser. They are thus of an active nature, but in contrast to the thermocouples they do not influence the flame. These methods deduce the temperature of a flame in conjunction with by taking account of the light emitted from the source and of the measuring volume.
A known optical, non-active temperature measurement is carried out by means of pyrometry, use being made of the blackbody radiation emitted by carbon black particles contained in the flame. However, it is a problem to apply pyrometric temperature-measuring systems to flames from gaseous fuels. The optical signal is very weak here because of the very low carbon black content. An additional difficulty in the signal analysis is that the temperature- and wavelength-dependent emissivity of the radiating carbon black particles is known only approximately, and, in conjunction with undesired absorption effects on the path to the detector, this impairs the accuracy of the method.
The installation of all known, optical temperature-measuring devices is performed at the smallest possible distance from a flame. For this purpose, the measuring sensors are arranged either at right angles to the flow direction of the fuel mixture next to the flame front in the combustion chamber, or they are located downstream of the burner in a front plate, the measuring sensors being aligned obliquely relative to the flame front.
It is particularly disadvantageous in the case of such an installation that, because of thermo-acoustic fluctuations in the combustion chamber, the flame does not burn at a fixed point but fluctuates in a region of the combustion chamber. A consequence of this is that the determination of the temperature using the measuring installation described is subject to error, since an individual flame plane cannot be continuously detected.